The present invention relates to an amplifier system with on-demand power supply boost.
Amplifier efficiency is the ratio of power supplied by an amplifier to a load to the power supplied to the amplifier. The less power dissipation in an amplifier, the better the efficiency of the amplifier. For conventional amplifier systems (Class AB) with fixed supplies and processing signals with high peak-to-average (PAR) and low percentage actuation, there is a significant amount of wasted power. One method to reduce power is the on-demand power supply boost system which uses Class H technology on a much lower power supply voltage. However, the conventional amplifier system with on-demand power supply boost still has unnecessary power dissipation.
FIG. 1 illustrates a conventional amplifier system with on-demand power supply boost. Assume supplies of +6V and −6V are applied to VCC & VEE respectively. An amplifier 101 receives varying input signals INP1 and INP2 at an input terminal 102 and is powered by VCCP from a charge pump 103A and VEEP from a charge pump 103B. The charge pump 103A includes a transistor QA, a diode DA, a capacitor CA, and a current source 1031.
The transistor QA is a NPN transistor and is biased as a voltage follower, with its base coupled to the varying input signals via a mixer 105 and a gain circuit 104A, its collector coupled to a fixed DC voltage VCC, and its emitter coupled to the current source 1031. Assume the diode drop across the base-emitter junction of the transistor QA is 0.5V, the input signal at its base varies between −5V and 5V, the drop across the diode DA is 0.5V, and the fixed DC voltage Vcc is 6V. When the input signal at the base of the transistor QA is −5V, the transistor QA is off. The voltage at the top part of the capacitor CA is 5.5V (the 6V of the fixed DC voltage VCC minus the 0.5V voltage drop over the diode DA), and the voltage at the bottom part of the capacitor CA is −6V (the current source 1031 is saturated causing the bottom plate of CA, or node CAPP, to be drawn to VEE or −6V). Thus the total voltage across the capacitor CA is 11.5V. When the input signal at the base of the transistor QA rises from −5V to 5V, the voltage at the bottom part of the capacitor CA changes from −6V to +3.5V. Since the voltage across the capacitor CA cannot change instantly, the additional 9.5V boosts the voltage at the top part of the capacitor CA from 5.5V to 15V. Thus the amplifier 101 is momentarily provided with a power supply voltage VCCP of 15V when needed although there is only a power supply Vcc of 6V available.
Similarly, the charge pump 103B includes a transistor QB, a diode DB, a capacitor CB, and a current source 1032, and provides to the amplifier 101 a power supply voltage VEEP switching between −5.5v and −15V. With this system, a dynamic supply of 30V is possible with only 12V total DC supply.
As shown in FIG. 2, the charge pumps 103A and 103B provide boost voltages VCCP and VEEP, respectively, to the amplifier 101 in excess of its power supply when the input peaks on the terminal 102 predict that output (VOPA and VONA) peaks will exceed the capacity of the power supply, and assure that there is sufficient power supply voltage to accommodate peak signals when necessary to avoid clipping and yet keep the voltage supply on a low level when the input is operating in its normal range of RMS voltage.
Gain circuits 104A and 104B are provided so that when the input signal peaks, the voltages VCCP and VEEP provided by charge pumps 103A and 103B will be sufficient to provide the necessary supply voltages to the amplifier 101 in view of the amplifier 101's particular gain. The mixer 105 ensures that the proper swing polarity reaches amplifiers 104A and 104B regardless of the swing polarity at the terminal 102.
Even with its power savings over conventional class AB systems, this on-demand amplifier system has unnecessary power dissipation, since transistors QA and QB work as voltage followers and remain on, and VCCP and VEEP vary continuously in correspondence with input signals at the terminal 102 even when the input does not peak (e.g., before t1 in FIG. 2) and it is unnecessary to compensate for excessive peaks. Therefore, it would be desirable to provide a more power efficient amplifier system with on-demand power supply boost.